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  <book id="debug-objects-guide">
   <bookinfo>
    <title>Debug objects life time</title>
  
    <authorgroup>
     <author>
      <firstname>Thomas</firstname>
      <surname>Gleixner</surname>
      <affiliation>
       <address>
        <email>tglx@linutronix.de</email>
       </address>
      </affiliation>
     </author>
    </authorgroup>
  
    <copyright>
     <year>2008</year>
     <holder>Thomas Gleixner</holder>
    </copyright>
  
    <legalnotice>
     <para>
       This documentation is free software; you can redistribute
       it and/or modify it under the terms of the GNU General Public
       License version 2 as published by the Free Software Foundation.
     </para>
  
     <para>
       This program is distributed in the hope that it will be
       useful, but WITHOUT ANY WARRANTY; without even the implied
       warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
       See the GNU General Public License for more details.
     </para>
  
     <para>
       You should have received a copy of the GNU General Public
       License along with this program; if not, write to the Free
       Software Foundation, Inc., 59 Temple Place, Suite 330, Boston,
       MA 02111-1307 USA
     </para>
  
     <para>
       For more details see the file COPYING in the source
       distribution of Linux.
     </para>
    </legalnotice>
   </bookinfo>
  
  <toc></toc>
  
    <chapter id="intro">
      <title>Introduction</title>
      <para>
        debugobjects is a generic infrastructure to track the life time
        of kernel objects and validate the operations on those.
      </para>
      <para>
        debugobjects is useful to check for the following error patterns:
  	<itemizedlist>
  	  <listitem><para>Activation of uninitialized objects</para></listitem>
  	  <listitem><para>Initialization of active objects</para></listitem>
  	  <listitem><para>Usage of freed/destroyed objects</para></listitem>
  	</itemizedlist>
      </para>
      <para>
        debugobjects is not changing the data structure of the real
        object so it can be compiled in with a minimal runtime impact
        and enabled on demand with a kernel command line option.
      </para>
    </chapter>
  
    <chapter id="howto">
      <title>Howto use debugobjects</title>
      <para>
        A kernel subsystem needs to provide a data structure which
        describes the object type and add calls into the debug code at
        appropriate places. The data structure to describe the object
        type needs at minimum the name of the object type. Optional
        functions can and should be provided to fixup detected problems
        so the kernel can continue to work and the debug information can
        be retrieved from a live system instead of hard core debugging
        with serial consoles and stack trace transcripts from the
        monitor.
      </para>
      <para>
        The debug calls provided by debugobjects are:
        <itemizedlist>
  	<listitem><para>debug_object_init</para></listitem>
  	<listitem><para>debug_object_init_on_stack</para></listitem>
  	<listitem><para>debug_object_activate</para></listitem>
  	<listitem><para>debug_object_deactivate</para></listitem>
  	<listitem><para>debug_object_destroy</para></listitem>
  	<listitem><para>debug_object_free</para></listitem>
  	<listitem><para>debug_object_assert_init</para></listitem>
        </itemizedlist>
        Each of these functions takes the address of the real object and
        a pointer to the object type specific debug description
        structure.
      </para>
      <para>
        Each detected error is reported in the statistics and a limited
        number of errors are printk'ed including a full stack trace.
      </para>
      <para>
        The statistics are available via /sys/kernel/debug/debug_objects/stats.
        They provide information about the number of warnings and the
        number of successful fixups along with information about the
        usage of the internal tracking objects and the state of the
        internal tracking objects pool.
      </para>
    </chapter>
    <chapter id="debugfunctions">
      <title>Debug functions</title>
      <sect1 id="prototypes">
        <title>Debug object function reference</title>
  !Elib/debugobjects.c
      </sect1>
      <sect1 id="debug_object_init">
        <title>debug_object_init</title>
        <para>
  	This function is called whenever the initialization function
  	of a real object is called.
        </para>
        <para>
  	When the real object is already tracked by debugobjects it is
  	checked, whether the object can be initialized.  Initializing
  	is not allowed for active and destroyed objects. When
  	debugobjects detects an error, then it calls the fixup_init
  	function of the object type description structure if provided
  	by the caller. The fixup function can correct the problem
  	before the real initialization of the object happens. E.g. it
  	can deactivate an active object in order to prevent damage to
  	the subsystem.
        </para>
        <para>
  	When the real object is not yet tracked by debugobjects,
  	debugobjects allocates a tracker object for the real object
  	and sets the tracker object state to ODEBUG_STATE_INIT. It
  	verifies that the object is not on the callers stack. If it is
  	on the callers stack then a limited number of warnings
  	including a full stack trace is printk'ed. The calling code
  	must use debug_object_init_on_stack() and remove the object
  	before leaving the function which allocated it. See next
  	section.
        </para>
      </sect1>
  
      <sect1 id="debug_object_init_on_stack">
        <title>debug_object_init_on_stack</title>
        <para>
  	This function is called whenever the initialization function
  	of a real object which resides on the stack is called.
        </para>
        <para>
  	When the real object is already tracked by debugobjects it is
  	checked, whether the object can be initialized. Initializing
  	is not allowed for active and destroyed objects. When
  	debugobjects detects an error, then it calls the fixup_init
  	function of the object type description structure if provided
  	by the caller. The fixup function can correct the problem
  	before the real initialization of the object happens. E.g. it
  	can deactivate an active object in order to prevent damage to
  	the subsystem.
        </para>
        <para>
  	When the real object is not yet tracked by debugobjects
  	debugobjects allocates a tracker object for the real object
  	and sets the tracker object state to ODEBUG_STATE_INIT. It
  	verifies that the object is on the callers stack.
        </para>
        <para>
  	An object which is on the stack must be removed from the
  	tracker by calling debug_object_free() before the function
  	which allocates the object returns. Otherwise we keep track of
  	stale objects.
        </para>
      </sect1>
  
      <sect1 id="debug_object_activate">
        <title>debug_object_activate</title>
        <para>
  	This function is called whenever the activation function of a
  	real object is called.
        </para>
        <para>
  	When the real object is already tracked by debugobjects it is
  	checked, whether the object can be activated.  Activating is
  	not allowed for active and destroyed objects. When
  	debugobjects detects an error, then it calls the
  	fixup_activate function of the object type description
  	structure if provided by the caller. The fixup function can
  	correct the problem before the real activation of the object
  	happens. E.g. it can deactivate an active object in order to
  	prevent damage to the subsystem.
        </para>
        <para>
  	When the real object is not yet tracked by debugobjects then
  	the fixup_activate function is called if available. This is
  	necessary to allow the legitimate activation of statically
  	allocated and initialized objects. The fixup function checks
  	whether the object is valid and calls the debug_objects_init()
  	function to initialize the tracking of this object.
        </para>
        <para>
  	When the activation is legitimate, then the state of the
  	associated tracker object is set to ODEBUG_STATE_ACTIVE.
        </para>
      </sect1>
  
      <sect1 id="debug_object_deactivate">
        <title>debug_object_deactivate</title>
        <para>
  	This function is called whenever the deactivation function of
  	a real object is called.
        </para>
        <para>
  	When the real object is tracked by debugobjects it is checked,
  	whether the object can be deactivated. Deactivating is not
  	allowed for untracked or destroyed objects.
        </para>
        <para>
  	When the deactivation is legitimate, then the state of the
  	associated tracker object is set to ODEBUG_STATE_INACTIVE.
        </para>
      </sect1>
  
      <sect1 id="debug_object_destroy">
        <title>debug_object_destroy</title>
        <para>
  	This function is called to mark an object destroyed. This is
  	useful to prevent the usage of invalid objects, which are
  	still available in memory: either statically allocated objects
  	or objects which are freed later.
        </para>
        <para>
  	When the real object is tracked by debugobjects it is checked,
  	whether the object can be destroyed. Destruction is not
  	allowed for active and destroyed objects. When debugobjects
  	detects an error, then it calls the fixup_destroy function of
  	the object type description structure if provided by the
  	caller. The fixup function can correct the problem before the
  	real destruction of the object happens. E.g. it can deactivate
  	an active object in order to prevent damage to the subsystem.
        </para>
        <para>
  	When the destruction is legitimate, then the state of the
  	associated tracker object is set to ODEBUG_STATE_DESTROYED.
        </para>
      </sect1>
  
      <sect1 id="debug_object_free">
        <title>debug_object_free</title>
        <para>
  	This function is called before an object is freed.
        </para>
        <para>
  	When the real object is tracked by debugobjects it is checked,
  	whether the object can be freed. Free is not allowed for
  	active objects. When debugobjects detects an error, then it
  	calls the fixup_free function of the object type description
  	structure if provided by the caller. The fixup function can
  	correct the problem before the real free of the object
  	happens. E.g. it can deactivate an active object in order to
  	prevent damage to the subsystem.
        </para>
        <para>
  	Note that debug_object_free removes the object from the
  	tracker. Later usage of the object is detected by the other
  	debug checks.
        </para>
      </sect1>
  
      <sect1 id="debug_object_assert_init">
        <title>debug_object_assert_init</title>
        <para>
  	This function is called to assert that an object has been
  	initialized.
        </para>
        <para>
  	When the real object is not tracked by debugobjects, it calls
  	fixup_assert_init of the object type description structure
  	provided by the caller, with the hardcoded object state
  	ODEBUG_NOT_AVAILABLE. The fixup function can correct the problem
  	by calling debug_object_init and other specific initializing
  	functions.
        </para>
        <para>
  	When the real object is already tracked by debugobjects it is
  	ignored.
        </para>
      </sect1>
    </chapter>
    <chapter id="fixupfunctions">
      <title>Fixup functions</title>
      <sect1 id="debug_obj_descr">
        <title>Debug object type description structure</title>
  !Iinclude/linux/debugobjects.h
      </sect1>
      <sect1 id="fixup_init">
        <title>fixup_init</title>
        <para>
  	This function is called from the debug code whenever a problem
  	in debug_object_init is detected. The function takes the
  	address of the object and the state which is currently
  	recorded in the tracker.
        </para>
        <para>
  	Called from debug_object_init when the object state is:
  	<itemizedlist>
  	  <listitem><para>ODEBUG_STATE_ACTIVE</para></listitem>
  	</itemizedlist>
        </para>
        <para>
  	The function returns 1 when the fixup was successful,
  	otherwise 0. The return value is used to update the
  	statistics.
        </para>
        <para>
  	Note, that the function needs to call the debug_object_init()
  	function again, after the damage has been repaired in order to
  	keep the state consistent.
        </para>
      </sect1>
  
      <sect1 id="fixup_activate">
        <title>fixup_activate</title>
        <para>
  	This function is called from the debug code whenever a problem
  	in debug_object_activate is detected.
        </para>
        <para>
  	Called from debug_object_activate when the object state is:
  	<itemizedlist>
  	  <listitem><para>ODEBUG_STATE_NOTAVAILABLE</para></listitem>
  	  <listitem><para>ODEBUG_STATE_ACTIVE</para></listitem>
  	</itemizedlist>
        </para>
        <para>
  	The function returns 1 when the fixup was successful,
  	otherwise 0. The return value is used to update the
  	statistics.
        </para>
        <para>
  	Note that the function needs to call the debug_object_activate()
  	function again after the damage has been repaired in order to
  	keep the state consistent.
        </para>
        <para>
  	The activation of statically initialized objects is a special
  	case. When debug_object_activate() has no tracked object for
  	this object address then fixup_activate() is called with
  	object state ODEBUG_STATE_NOTAVAILABLE. The fixup function
  	needs to check whether this is a legitimate case of a
  	statically initialized object or not. In case it is it calls
  	debug_object_init() and debug_object_activate() to make the
  	object known to the tracker and marked active. In this case
  	the function should return 0 because this is not a real fixup.
        </para>
      </sect1>
  
      <sect1 id="fixup_destroy">
        <title>fixup_destroy</title>
        <para>
  	This function is called from the debug code whenever a problem
  	in debug_object_destroy is detected.
        </para>
        <para>
  	Called from debug_object_destroy when the object state is:
  	<itemizedlist>
  	  <listitem><para>ODEBUG_STATE_ACTIVE</para></listitem>
  	</itemizedlist>
        </para>
        <para>
  	The function returns 1 when the fixup was successful,
  	otherwise 0. The return value is used to update the
  	statistics.
        </para>
      </sect1>
      <sect1 id="fixup_free">
        <title>fixup_free</title>
        <para>
  	This function is called from the debug code whenever a problem
  	in debug_object_free is detected. Further it can be called
  	from the debug checks in kfree/vfree, when an active object is
  	detected from the debug_check_no_obj_freed() sanity checks.
        </para>
        <para>
  	Called from debug_object_free() or debug_check_no_obj_freed()
  	when the object state is:
  	<itemizedlist>
  	  <listitem><para>ODEBUG_STATE_ACTIVE</para></listitem>
  	</itemizedlist>
        </para>
        <para>
  	The function returns 1 when the fixup was successful,
  	otherwise 0. The return value is used to update the
  	statistics.
        </para>
      </sect1>
      <sect1 id="fixup_assert_init">
        <title>fixup_assert_init</title>
        <para>
  	This function is called from the debug code whenever a problem
  	in debug_object_assert_init is detected.
        </para>
        <para>
  	Called from debug_object_assert_init() with a hardcoded state
  	ODEBUG_STATE_NOTAVAILABLE when the object is not found in the
  	debug bucket.
        </para>
        <para>
  	The function returns 1 when the fixup was successful,
  	otherwise 0. The return value is used to update the
  	statistics.
        </para>
        <para>
  	Note, this function should make sure debug_object_init() is
  	called before returning.
        </para>
        <para>
  	The handling of statically initialized objects is a special
  	case. The fixup function should check if this is a legitimate
  	case of a statically initialized object or not. In this case only
  	debug_object_init() should be called to make the object known to
  	the tracker. Then the function should return 0 because this is not
  	a real fixup.
        </para>
      </sect1>
    </chapter>
    <chapter id="bugs">
      <title>Known Bugs And Assumptions</title>
      <para>
  	None (knock on wood).
      </para>
    </chapter>
  </book>